CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Pressure averaging at the outlets.

Register Blogs Community New Posts Updated Threads Search

Like Tree4Likes
  • 2 Post By CycLone
  • 1 Post By AtoHM
  • 1 Post By Gert-Jan

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 21, 2007, 13:47
Default Pressure averaging at the outlets.
  #1
KM
Guest
 
Posts: n/a
Hi there,

For pressure specified outlet boundary conditions, fluent has one option 'Radial equilibrium pressure distribution'. While CFX-11 seems to have two options for the Pressure Averaging at the pressure outlets. 1) Average Over Whole Outlet 2) Circumferential.

Is 'Circumferential averaging' in CFX equivalent to 'Radial equilibrium pressure distribution' in Fluent?

I woould be very much thankful to you for your help !

Many Thanks,

Best Regards, KM.
  Reply With Quote

Old   November 21, 2007, 17:42
Default Re: Pressure averaging at the outlets.
  #2
CycLone
Guest
 
Posts: n/a
Hi KM,

With a Radial Equilibrium boundary condition, the specified pressure applies only to the position of minimum radius (relative to the axis of rotation) at the boundary. The static pressure on the rest of the zone is calculated from the assumption that radial velocity is negligible, so that the pressure gradient is given by

dp/dr = rho*v_theta^2/r

The Average Static Pressure boundary condition solves for the static pressure implicitely and will acheive an equilibrium static pressure profile. In the limit of zero radial velocity and swirling flow this is equivalent to radial equilibrium. A radial equilibrium static pressure profile is better than a fixed static pressure at swirling outlets, but it is still explicit and will result in a non-physical static pressure profile at the outlet. The CFX equivalent would be to specify the Static Pressure as an radial equilibrium expression (i.e. integrate the above equation), but I would suggest using the Average Static Pressure condition instead.

Other options: 1) Average over the whole outlet: The pressure is allowed to float over the whole outlet, or you can specify a specific region over which to evaluate the area averaged static pressure. There is no equivalent of this in Fluent

2) Circumferential: If this option is picked you will need to specify a one dimenstional spanwise profile. The solver will match/fix the circumferentially averaged pressure at each location, but allows the circumferential profile to float. A single value just means the circumferentially averaged profile is constant. There is no equivalent to this in Fluent.

Note that similar options are available for Mass Flow specified outlets. The default for Mass Flow outlets is also to let the static pressure and local mass flux to float, but match the specified mass flow. Options are also available to shift a pressure profile at a mass flow outlet, circumferentially match a pressure profile, etc.

-CycLone
fumiya and Mazze[ITA] like this.
  Reply With Quote

Old   November 23, 2007, 14:50
Default Re: Pressure averaging at the outlets.
  #3
KM
Guest
 
Posts: n/a
Dear CycLone, Thank you very much for helping me again with your detailed reply. It helps a lot. Many Thanks, KM.
  Reply With Quote

Old   April 1, 2021, 00:25
Default Hello Cyclone I have my radial velocity plot between stationary and rotating rings
  #4
New Member
 
Rami A
Join Date: Nov 2020
Posts: 2
Rep Power: 0
Neno is on a distinguished road
Please see the image attached. My solution is converged but the chart plot of the radial velocity doesn't make sense. the plot should reach zero near both walls due to the non-slip condition.
Attached Images
File Type: png Screen Shot 2021-03-31 at 2.28.39 PM.png (82.4 KB, 31 views)
Neno is offline   Reply With Quote

Old   April 1, 2021, 02:05
Default
  #5
Senior Member
 
M
Join Date: Dec 2017
Posts: 698
Rep Power: 12
AtoHM is on a distinguished road
Might be caused by this
https://www.cfd-online.com/Wiki/Ansy...t_the_walls.3F
Opaque likes this.
AtoHM is offline   Reply With Quote

Old   April 1, 2021, 06:24
Default
  #6
Senior Member
 
Join Date: Jun 2009
Posts: 1,869
Rep Power: 33
Opaque will become famous soon enough
You can overlay your mesh on top of the radial profile and see how coarse the mesh is near the boundary.

If you plot the same profile using "hybrid" values for velocities, how does the profile look like?
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
Opaque is offline   Reply With Quote

Old   April 1, 2021, 07:28
Default
  #7
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,910
Rep Power: 28
Gert-Jan will become famous soon enough
You mention that you have a rotating ring. Assuming that you plot Hybrid values, isn't the value you see on the wall, the local velocity of the rotating part?
Also, make sure what velocity you are looking at. Is it Velocity or Velocity in Stn Frame?

In other words: there are various way of presenting the results. Make sure that you know what you are doing and take the one you need.
Opaque likes this.
Gert-Jan is offline   Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pulsatile pressure inlet with pressure outlet a.lynchy FLUENT 3 March 23, 2012 14:45
UDF to define or adjust pressure??? engahmed FLUENT 0 July 6, 2010 18:19
pressure BC-natural convection engahmed Main CFD Forum 0 June 16, 2010 12:38
Neumann pressure BC and velocity field Antech Main CFD Forum 0 April 25, 2006 03:15
FLOW AROUND A PLATE_NEGATIVE ABSOLUTE PRESSURE???? tania FLUENT 11 March 23, 2004 09:51


All times are GMT -4. The time now is 12:26.